1. Field of the Invention
The present invention relates to fiber optical temperature sensors, and more particularly to such sensors which are based on the ability of certain materials to change color in dependence on temperature. This applies not only to visible light, but generally also to non-visible light such as ultraviolet and infrared radiation.
2. Prior Art
In many applications of temperature measurement, it is of great interest to be able to use a non-electrical method involving light transmission through optical fibers. Such applications are particularly advantageous in explosive environments or in areas with strong electrical or magnetic fields. It is known to use fiber optical thermometers, where the temperature-dependent expansion of a body is allowed to influence light transmission in a fiber optical system. In such a known sensor, light is reflected from a fiber transmitting light against the concave surface of a fluid located in a container, the fluid being subjected to the temperature to be measured. Through temperature variations the distance of the concave surface from the fiber end is changed and influences the light in a fiber extending from the sensor.
A sensor of the above-mentioned kind has at least the disadvantage that it cannot be located at any random place without taking special measures. Another disadvantage is that it is not possible to distinguish the change in light transmission, caused by the temperature, from other attenuations which may occur. Such undesirable attenuations may occur if the fiber is bent, or by a joint in the fiber. For example, a fiber having a diameter of 400 microns may show a transmission loss of 40% in case of a bend with a radius of 10 to 30 mm. In practice, therefore, such a sensor must be calibrated with a known temperature after it has been positioned in the measuring object. This can be difficult or even impossible, for example inside the winding in an electrical machine or transformer. This disadvantage becomes even more pronounced where the optical fiber may be subjected to uncontrollable deformation during the measurement.